The invention generally relates to a cooling method and apparatus for use with a fuel cell stack.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), a membrane that may permit only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is oxidized to produce hydrogen protons that pass through the PEM. The electrons produced by this oxidation travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions may be described by the following equations:
H2xe2x86x922H++2exe2x88x92 at the anode of the cell,
and
O2+4H++4exe2x88x92xe2x86x922H2O at the cathode of the cell.
Because a single fuel cell typically produces a relatively small voltage (around 1 volt, for example), several serially connected fuel cells may be formed out of an arrangement called a fuel cell stack to produce a higher voltage. The fuel cell stack may include different plates that are stacked one on top of the other in the appropriate order, and each plate may be associated with more than one fuel cell of the stack. The plates may be made from a graphite composite material or metal and may include various channels and orifices to, as examples, route the above-described reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. In addition to the membrane, a catalyst and gas diffusion layer are sandwiched between the anode and cathode plates. The catalyst layer may be placed on the membrane or on the gas diffusion layer. The gas diffusion layer may be made out of an electrically conductive and porous diffusion material, such as a carbon cloth or paper material, for example.
Referring to FIG. 1, as an example, a fuel cell stack 10 may be formed out of repeating units called plate modules 12. In this manner, each plate module 12 includes a set of composite plates that may form several fuel cells. For example, for the arrangement depicted in FIG. 1, an exemplary plate module 12a may be formed from a cathode cooler plate 14, a bipolar plate 16, an anode cooler plate 18, a cathode cooler plate 20, a bipolar plate 22 and an anode cooler plate 24 that are stacked from bottom to top (as depicted in FIG. 1) in the listed order. The (anode or cathode) cooler plate functions as a heat exchanger by routing a coolant through flow channels in either the upper or lower surface of the cooler plate to remove heat from the stack 10. The surface of the cooler plate that is not used to route the coolant includes flow channels to communicate either hydrogen (for the anode cooler plates 18 and 24) or air (that provides the oxygen for the cathode cooler plates 14 and 20) to an associated fuel cell. The bipolar plates 16 and 22 include flow channels on one surface (i.e., on the top or bottom surface) to route hydrogen to an associated fuel cell and flow channels on the opposing surface to route oxygen to another associated fuel cell. Due to this arrangement, each fuel cell may be formed in part from one bipolar plate and one cooler plate, for example.
As an example, one fuel cell of the plate module 12a may include an anode-membrane-cathode sandwich, called a membrane-electrode-assembly (MEA), that is located between the anode cooler plate 24 and the bipolar plate 22. In this manner, the upper surface of the bipolar plate 22 includes flow channels to communicate oxygen near the cathode of the MEA, and the lower surface of the anode cooler plate 24 includes flow channels to communicate hydrogen near the anode of the MEA.
As another example, another fuel cell of the plate module 12a may be formed from another MEA that is located between the bipolar plate 22 and the cathode cooler plate 20. The lower surface of the bipolar plate 22 includes flow channels to communicate hydrogen near the anode of the MEA, and the upper surface of the cathode cooler plate 20 includes flow channels to communicate air near the cathode of the MEA. The other fuel cells of the plate module 12a may be formed in a similar manner. To communicate the hydrogen, oxygen and coolant to/from the various flow channels of the stack 10, the plates include openings that align to form passageways of a reactant and coolant manifold.
The coolant typically remains in its liquid state to remove heat from the fuel cell stack 10. As an example, the coolant may be de-ionized water, a fluid that has desirable heat transport properties. However, when the fuel cell stack is used to provide power to a house (for example) the fuel cell stack may reside outside of the house, and thus, the de-ionized water may freeze in colder climates. Therefore, a coolant that has a freezing point temperature that is lower than the freezing point temperature of de-ionized water may be used. This coolant may not remove heat from the stack as efficiently as de-ionized water. As a result, an increased pumping rate (as compared to the pumping rate that is used with de-ionized water) may be needed to maintain the same heat rejection rate and fluid delta temperature that are provided by the de-ionized water. This increased pumping rate, in turn, may increase parasitic losses in the fuel cell system and thus, decrease the overall efficiency of the system.
Thus, there is a continuing need for a system that addresses one or more of the above-stated problems.
In an embodiment of the invention, a method includes using different states of a coolant to establish a thermal siphon to circulate the coolant through a fuel cell stack. The coolant has a boiling point temperature that is near a predetermined operating temperature of the fuel cell stack.
In another embodiment of the invention, a system includes a stack of fuel cell flow plates and a condenser. The stack of fuel cell flow plates include openings to form an inlet manifold passageway and an outlet manifold passageway to communicate a coolant through the stack. The flow plates are capable of transferring thermal energy to the coolant to cause the coolant to change from a liquid state into a gas state. The condenser changes the coolant from the gas state to the liquid state. At least one conduit of the system is connected to communicate the coolant between the condenser and the inlet and outlet manifold passageways.
The advantages of the invention may include one or more of the following: the system may maintain a substantially uniform cell temperature across the entire active fuel cell area; the system may be self-regulating and thus, may not require an active control; no liquid pump may be required to circulate the coolant through the fuel cell stack; the system may operate over a wide range of ambient temperatures while maintaining a relatively constant fuel cell temperature; the system may have a low operating cost; and the system may be more efficient than conventional systems.
Advantages and other features of the invention will become apparent from the following description, from the drawing and from the claims.